US Defense Secretary Ashton Carter will depart Tuesday for an Asian security summit in Singapore, where Beijing’s military expansion across the South China Sea likely will once again dominate discussions. Regional neighbors are fretting over what they see as China’s expansionism as it rushes to exert sovereignty over the waterway, a major global shipping route believed to be home to large oil and gas reserves. China is using dredgers and other tools to convert low-lying ocean features and sandy blips into military bases.